To prepare a tire for retread the tire is buffed, an operation in which rasps strip the worn tread from the crown as the tire rotates on a fixture. A rubber layer of a desired thickness is left on the crown to maintain the integrity of the carcass plies and belts and provide a surface on to which to bond the new tread. The rasps provide an evenly contoured surface usually having a radius that peaks at the center of the tread.
The buffer rasps must avoid exposing or cutting into the underlying steel belts, which can damage the belts and destroy the integrity of the carcass, rendering the tire unfit for further use.
A retread facility will typically handle tires from a variety of manufacturers, which presents to the buffer different tire structures with different tread thicknesses and different belt constructions, shapes and locations. In addition, tires requiring retread are generally not worn to a standard condition. Further, an individual tire is not likely to have a uniform pattern of wear over the tread surface. Thus, the buffer and the buffer operator are required to determine essentially on a tire-by-tire basis the amount of rubber to remove from each tire.
Automated buffing machines typically rely on a sensor to measure the depth of tread rubber between the outer surface and the steel belts. The rasps, which remove rubber from the tire, are controlled based on a signal generated by the sensor. In conventional buffing machines, the sensor is fixed at the equator of the tire, that is, the central plane perpendicular to the rotation axis. This arrangement provides depth information that is accurate for the central plane, but not necessarily for the lateral regions of the tread.
The lack of information at the lateral or shoulder regions creates difficulties for buffing those areas. Steel belts in a tire are not always at the same depth across the tire tread. In addition, variations within manufacturing tolerance can produce tires with slight differences in belt location, particularly at the belt edges. Moreover, the belts may not follow the contour that the buffer will apply to the carcass crown.
A buffer relying on a centrally-located sensor, therefore, may sense sufficient rubber thickness at the center while the rasps cut into the steel belts at the shoulder of the tire, thus destroying the tire.
The present invention provides a solution to this problem with an apparatus for a tread depth monitoring that measures the rubber thickness across the width of the tire.
According to the invention, a tread depth sensor is carried on a mechanism that traverses the width of the tire in a reciprocating manner while the tire rotates to sense rubber depth across the width of the crown. Using depth information for the full width of the tire, a buffer can be controlled to stop with a minimum thickness of rubber for the entire surface.
A device in accordance with the invention includes a drive wheel that engages the tire tread surface and is driven by rotation of the tire. Drive wheel power is transmitted to a reciprocating motion device that transforms the rotation movement into linear reciprocating movement. A sensor is mounted to the reciprocating motion device to be carried transversely across the tire tread in reciprocating movements.
According to another aspect of the invention, a guide wheel attached to the sensor contacts the crown surface and follows the surface to guide the carrier movements on the crown. The guide wheel establishes a reference location for the sensor relative to the crown surface.
According to yet another aspect of the invention, the sensor and guide wheel are mounted to the reciprocating motion device for free movements normal, or perpendicular, to the tire crown surface to accommodate variations in the surface. Preferably, a vertical slide is used to mount the sensor and guide wheel for movements relative to the reciprocating motion device. As the guide wheel follows the crown, perpendicular movement of the guide wheel and sensor accommodate changes in the crown surface to prevent jarring or bouncing the sensor.
The tire sensor may be any suitable sensor for sensing the location of the steel belts under the crown rubber, for example, a magnetic sensor, an ultrasound sensor, a laser, or an optical device.